This invention relates to a semiconductor memory device.
As nonvolatile memories or read only memories (ROMs), an electrically erasable and programable read only memory (EEPROM) and an electrically programable read only memory (EPROM) have been utilized. These memories have disadvantages such as .circle.1 such as the writing time is long and .circle.2 the writing voltage is high, and .circle.3 a two-power-source system is needed. An EEPROM and EPROM of 16 kb is an example. Two power sources of different voltages are required, (25 V) necessary for a programing mode and (5 V) necessary for an access mode. Regarding the writing time, a long time of about 100 seconds is taken for writing all the bits. In addition, the access time is as long as 450 ns, which is about 4 times the access time of a random access memory (RAM) having an equal capacity. This is attributed to the fact that, because of the desire to reduce the cell size, the ROM performs a static operation by the use of only one data line. The EPROM further has the disadvantage that its memory content cannot be electrically erased.
On the other hand, the random access memory (RAM) has a memory capable of writing and reading at high speed in contrast to the read only memory (ROM). Although the memory cell of the RAM has a single data line as in the ROM, the operation thereof is dynamic and various signals can be generated in parellel. Since data can be sensed and amplified at high speed with the signals, the operating speed can be enhanced. When a memory of 16 kb is taken as an example, the access time of the RAM is about 1/4 that of the ROM. In addition, the writing time is substantially the same as the access time.
RAMs have the disadvantage that they are volatile memories. More specifically, in case of a dynamic RAM, a one-transistor MOS structure is currently used. With this structure, unless the memory is refreshed once in 2 msec., the memory content cannot be retained. When a power supply has been turned "off", the memory content naturally disappears. In case of a static RAM whose memory cell is a flip-flop structure, the refresh becomes unnecessary. However, when a power supply has been turned "off" to permit no current to be supplied to the memory cell, a memory content cannot be retained as in the dynamic RAM.